Activated carbon fiber is produced by carbonizing a raw material such as acrylic fiber, cellulosic fiber, phenol resin fiber, or pitch-based fiber and subsequently activating the carbonized fiber. The activated carbon fiber produced in the form of tow, woven fabrics, or felt is used as an adsorbent or a filter in devices such as those used for filtering automobile gasoline, deodorizing tobacco smoke, recovering a solvent, or purifying water. Due to its excellent performance the activated carbon fiber has attracted increasing attention. Particularly the activated carbon fiber derived from acrylic fiber as the raw material promises application to a great diversity of uses owing to its specific adsorbing ability and excellent mechanical strength due to the nitrogen atoms contained therein.
An example of a method of manufacturing activated carbon fiber from acrylic fiber as the raw material involves preoxidizing the acrylic fiber in an oxidative atmosphere such as air to obtain a preoxidized fiber and subsequently activating the preoxidized fiber with an activating medium such as steam. This method is taught by U.S. Pat. Nos. 4,256,607 and 4,285,831.
In the conventional method, preoxidation is first carried out at 200.degree. to 400.degree. C. Subsequently the activation is carried out at a temperature generally above 600.degree. C. and particularly in the neighborhood of 800.degree. C. When the activation is carried out at higher temperatures, the time for the treatment is shortened and the efficiency of the overall process is consequently improved. In accordance with the conventional method, if the activation temperature is elevated to level exceeding 1,000.degree. C., for example, the fiber is suddenly shrunk, and may be shrunk to the extent of sustaining breakage and embrittlement. This renders the commercial production of activated carbon fiber infeasible. Accordingly, it is generally believed that it is not possible to manufacture activated carbon fiber of high quality by high-temperature activation.
The present inventors have diligently studied the steps of preoxidation and activation in order to determine the conditions which, contrary to the generally accepted concept, make it possible for activation to be effectively performed at higher temperatures. They have consequently succeeded in developing a technique which is capable of producing high quality activated carbon fiber, in spite of the fact that the activation is carried out at an elevated temperature for a shortened period of time.